The most common and most likely answer to this crossword clue is the 11 letter word RING-FENCED
I've seen this crossword clue in The Guardian
R
I
N
G
-
F
E
N
C
E
D
Buddy explains!
The straight part
We know that "Reserved" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reserved = ring-fenced.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does RING-FENCED mean?
RING-FENCED verb- To guarantee the safety of funds or investment. In particular, to ensure some degree of distinction for monies, that may provide immunity against untoward financial claims.
- To specify that funds may only be used for a specific purpose, especially government funds.
- To require workers to remain in (particular types of work), as during wartime.